New evaluation of thermal neutron scattering libraries for light and heavy water

José Ignacio Márquez Damián ,J.r Granada ,F Cantargi ,D Roubtsov

doi:10.1051/epjconf/201714613001

Abstract

In order to improve the design and safety of thermal nuclear reactors and for verification of criticality safety conditions on systems with significant amount of fissile materials and water, it is necessary to perform high-precision neutron transport calculations and estimate uncertainties of the results. These calculations are based on neutron interaction data distributed in evaluated nuclear data libraries. To improve the evaluations of thermal scattering sub-libraries, we developed a set of thermal neutron scattering cross sections (scattering kernels) for hydrogen bound in light water, and deuterium and oxygen bound in heavy water, in the ENDF-6 format from room temperature up to the critical temperatures of molecular liquids. The new evaluations were generated and processable with NJOY99 and also with NJOY-2012 with minor modifications (updates), and with the new version of NJOY-2016. The new TSL libraries are based on molecular dynamics simulations with GROMACS and recent experimental data, and result in an improvement of the calculation of single neutron scattering quantities. In this work, we discuss the importance of taking into account self-diffusion in liquids to accurately describe the neutron scattering at low neutron energies (quasi-elastic peak problem). To improve modeling of heavy water, it is important to take into account temperature-dependent static structure factors and apply Skold approximation to the coherent inelastic components of the scattering matrix. The usage of the new set of scattering matrices and cross-sections improves the calculation of thermal critical systems moderated and/or reflected with light/heavy water obtained from the International Criticality Safety Benchmark Evaluation Project (ICSBEP) handbook. For example, the use of the new thermal scattering library for heavy water, combined with the ROSFOND-2010 evaluation of the cross sections for deuterium, results in an improvement of the C/E ratio in 48 out of 65 international benchmark cases calculated with the Monte Carlo code MCNP5, in comparison with the existing library based on the ENDF/B-VII.0 evaluation.

Highlights

The scattering of low energy neutrons with matter is described by the double differential scattering cross section
The new evaluations for light and heavy water are based on the CAB Model, which is an improvement over two older water thermal scattering evaluations: the General Atomics Model [4] and the IKE Model [5]
Compared with the General Atomics model used in ENDF/B-III to ENDF/B-VI, and the IKE model used in ENDF/B-VII and JEFF 3.2, the CAB Model introduces three main changes:

Summary

Introduction

The scattering of low energy neutrons with matter is described by the double differential scattering cross section. For scatterers without low energy resonances, the double differential cross section can be factorized as:. 2kT E where E and E are the incident and secondary energies, μ is the cosine of the scattering angle in the laboratory system, σb is the bound atom scattering cross section, kT is the temperature in eV and S is the so called thermal scattering law. In the evaluated nuclear data libraries the scattering law is tabulated over a grid of α and β values in the ENDF-6 format [1], and processing codes like NJOY [2] or GRUCON [3] are used to reconstruct the double differential scattering cross section using Eqs. In this paper we briefly introduce the models used to evaluate the new libraries, and summarize the validation data collected since the libraries were first introduced in 2014

Models

Experimental validation and benchmarking

Conclusions